sunxi: defconfig: enable eMMC boot partition support

Now that the SPL can safely detect whether it was loaded from an eMMC
boot partition or the normal user data partition, let's enable this
feature on some boards that feature eMMC storage.

That covers the boards where I could test this on, and allows the same
build to be written to an SD card, eMMC user partition, eMMC boot
partition, or into SPI NOR flash.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>